One of the first places that penicillin ended up being used was to cure the rampant STD infections among US troops. The infections were a serious problem for armies in the pre-antibiotic era, which produced some pretty amazing propaganda aimed at persuading soldiers to remain chaste, or at the very least, use a condom. These seem quaint to us now, but only because antibiotics defanged the most pernicious diseases. (Obviously, the surge of AIDS changed that calculation--but AIDS is actually relatively hard to get.)
Less than a century after we conquered syphilis and gonorrhea, the CDC warns that 100 percent antibiotic resistance is on its way:
Gonorrhea, one of the most common sexually transmitted diseases in the United States, is increasingly showing resistance to one of the last known effective antibiotic treatments, leading researchers from the Centers for Disease Control to "sound the alarm" about potentially untreatable forms of the disease.
That's serious stuff. Untreated gonorrhea is extremely unpleasant, and can have awful long-term side effects like infertility, ectopic pregnancy, and bladder cancer (in men).
This does make one wonder: how will modern sexual customs change if untreatable STDs once again become common? We think of the sexual revolution as having been sparked by the pill, and of course, that was a large part of it. But I doubt the pill would have been popular if antibiotics hadn't already taken care of the diseases that used to afflict the promiscuous before World War II.
I'm not arguing that we'd return to pre-sixties morality--obviously, AIDS did not cause the gay community to stop having sex. On the other hand, given drug-development timelines, we actually developed treatments pretty rapidly (and very actively managed them to prevent resistance--that's one of the reasons that patients take "cocktails" instead of single drugs). People have gotten used to thinking of pharmaceutical development as a wonder-drug factory that can pop out treatments on demand, provided that we want it badly enough. But that's not actually how it works, particularly with antibiotics. Once chlamydia or gonorrhea develop resistance to the antibiotics we have, there's no guarantee that we'll get new ones that treat them. And safe sex never became as common as educators and activists had hoped.